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I was wondering at what age children develop a concept of God and also of mortality.

With DS I don't know about God, but he was 4 when he developed the concept of mortality. It wasn't a pleasant time for him, or me, as he agonised over the thought that I might die and leave him. It lasted about 6-12 months, but he would repeatedly say to me "If you die I want to die too. I don't want to be left here without you".